fn c_mut2() -> impl for<'a> FnMut(&'a str) -> String {
    let s = "hello".to_string();
    move |i| {
        let mut c = s.clone();
        c += i;
        c
    }
}

#[cfg(test)]
mod tests {
    use crate::c_mut2;

    #[test]
    fn it_works() {
        let i = "world";
        let mut arr_closure = c_mut2();
        assert_eq!(arr_closure(i), "helloworld");
    }
}
